I picked up an 81 GL 1100 and have been going over things and performing some neglected maintenance. It is supposed to be a low mileage bike, and cosmetically is in good shape. I replaced the belts, thermostat,synced the carbs, serviced the final drive and rebuilt the forks, calipers, etc etc. I haven't set the valves yet, just waiting to get a couple hundred miles on the bike and set the valves with the oil change.

I keep reading about people really jumping on the throttle, and getting up near redline. This machine runs strong, pulls pretty hard, up to about 5000 RPM. But at about 5000, it suddenly swoons and sounds like it is about to come apart. I don't really understand this, because it is smooth, and sounds happy from 2500 to 5000. The idle is steady,(initially had a lean miss on #1 at idle) and acceleration is decent right up to that mark. Any ideas what might be looking at here?

3 things to look at.
1..With a mirror and light in the plenum box,Rap the throttle and verify each slide rises and falls.Just one sticking is all it takes to dog the engine out.
2..Check the vacuum advance canister.be sure it holds a vacuum and you can draw a vacuum on it and hear the rpm change when it changes the timing.
Ignition is pre-set at 10*btdc and goes to (around 23* btdc) mechanically ,then vacuum hauls it up to 34* btdc*
3..A less likely problem is the exhaust header.I have seen one with a swollen inner thermal liner.This effectively restricts exhaust flow.

I gotta say I'm surprised. I fueled up and the tank hissed at me. So I took it back on the highway with the cap a half turn loose. Like a scalded cat she did run! One of the few things I hadn't replaced.